:: Survey Basics Laying out a Trench and Levelling. :: July 2005 :: Compiled by British Archaeological Jobs & Resources

Transcription

1 :: Survey Basics Laying out a Trench and Levelling :: July 2005 :: Compiled by British Archaeological Jobs & Resources

2 Survey Basics Laying out a trench and Levelling. Introduction One of the first tasks when preparing a site for excavation is laying out of the trenches in the locations decided previously. A rectangular or square trench has become the standard within the archaeological disciple and it is very easy to achieve a perfect trench with right angled corners every time. This knowledge is of course transferable to other aspects of survey, and therefore should be seen as a basic skill that must be understood and mastered. Along with this, it is certain that use of a dumpy level will be another core skill that is required by all archaeologists. This guide will provide you with both these skills.

3 Setting out a Trench Start with your fixed point, a single peg that is to be the corner of your first trench. Now attach two tapes to the nail on the top of the peg (or survey arrow). This is where Pythagoras' theorem comes in in geometry, a 2 + b 2 = c 2 Other useful diagonals are listed below, but you can now find the diagonal of any right angled triangle by using this method. You will also be able now to offset from a baseline tape using this method to arrive at a right angle Based on simple geometry, a right angle can be created by making the tapes work together as a triangle with sides of exactly 3, 4 and 5 metres. 8m 4m Pull one tape tight and hold at 4m, then with the other tape, play out 8m of tape Step 1 (the sum of the two remaining sides) and then hold the 8m mark over the other tape at 4m. Step 2 Step 1 Step 2 4m Now hold the long (8m length) tape at 3m and pull taught until both sides are tight - Step 3. You now have your third peg, and if you repeat this on the other side, you will now have four pegs which are all at right angles. Step 4 Step 3 Step 4 3m 5m 5m 8m length 4m 3m

4 When the long side (the hypotenuse) is 5 (for example), 3 and 4 (in this case) will be the lengths of the two shorter sides and therefore you have therefore created a right angle. Vertical heights, such as slopes, can be measured using tapes and measuring rods in a step-like fashion. The most useful angles to know are: 1m x 1m square 1.414m Diagonal 2m x 2m square 2.83m Diagonal 5m x 5m square 7.07m Diagonal 10m x 10m square 14.14m Diagonal 3m x 4m square 5m Diagonal Remember that you can always extend a line, so although you might use a 3x4m triangle to create a right angle, the lines can extend beyond this. extended line 5m 3m

5 Levelling The other basic skill, is easy to learn, and once again it is simple geometry. First find or establish your datum (a fixed point of a known height that all other measurements are taken from.) This can be an Ordnance bench mark, such as the one on the left. Or a temporary datum that you assign an arbitrary height number to though this should be tied into the OS datum as soon as possible. This is just one of the measures that keeps us all consistent and having to recalculate all your levels, when you have a real height value is not a pleasant task. Most maps will show you the location of Bench Marks establish them as early as possible as this will save having to redo all your level calculations. Now set up your dumpy level. It will probably be like the one pictured, and has an eyepiece, bullseye spirit level and 3 levelling screws as well as a focus for the telescope lens quite often the base has a 360 degree compass, for laying out, however as you now know how to create perfect right angles, this is not usually needed. Eyepiece Focus Instrument Focus Eyepiece Focus Instrument Focus Eyepiece Eyepiece Instrument Rotate 360 Rotate Levelling Plate Baseplate screw & plumbob hook Start by attaching the dumpy level to the legs and tighten the baseplate screw. Next turn the dumpy so it is parallel with two of the levelling screws. Turn BOTH screws either away from each other or towards each other simultaneously until the bubble in the spirit level is centralised. Now turn the dumpy 90 degrees so the level now points towards the levelling screw that has not been touched and then use this to complete the bubble levelling by putting the bubble in the middle of the bulls-eye. This should now result in the dumpy being perfectly level, which is very important, before you start to take spot heights.

6